Add optional default string for flInput.

[ericu]

Apologies if you have a preferred naming scheme for variables; I'm still learning Haskell style.

This adds an optional default string for flInput, without dealing with the complexity of the varargs stuff. Users can certainly compute their own string before passing it in, so this seems like decent bang for the buck.

[deech]

Looks great! Do you mind changing maybeDefault to defaultMsg or something like that?

[deech]

Thanks!